1. Field of Invention
Embodiments of the present invention relate generally to superconductors and methods for manufacturing the same. More specifically, embodiments of the present invention relate to a superconductor with high magnetic field properties, and to a method for enhancing high magnetic field properties of a superconductor.
2. Description of the Related Art
Currently Nb—Ti alloy and Nb3Sn are mostly used to produce high field magnets. Recently discovered 39K superconductor MgB2 has great potential for magnets, which has been intensively investigated (see C. Buzea and T. Yamashita, “Review of the Superconducting Properties of MgB2”, Supercond. Sci. Technol. 15, R115-R146 (2001)). For power transmission superconducting cables, made of the high Tc cuprates, are under development (see W. Buckel and R. Kleiner, “Superconductivity”, Wiley-VCH, Weinheim (2004), p. 382).
For magnet applications and power transmission, superconducting materials should have high critical current densities at high magnetic fields, due to strong vortex pinning or flux pinning. Nb—Ti alloy and Nb3Sn show good pinning properties. Nevertheless, for special scientific applications, such as accelerator applications, the high field properties of these materials may need to be enhanced more. Furthermore, MgB2, which can be used near 20K, does not exhibit good pinning properties.
Pinning properties of superconducting materials can be enhanced by adding impurities, columnar defects, artificial pins, and nano-particles, mechanical alloying, introducing grain boundaries and precipitates, and applying radiation damage (see W. Buckel and R. Kleiner, “Superconductivity”, Wiley-VCH, Weinheim (2004), p. 282). Changing the preparation conditions of samples also induces the disorder, leading to the enhancement of high field properties. These methods basically introduce the flux pinning centers into superconductors without reducing the critical temperature Tc significantly. Most progress has been made empirically, by trial and error, because there is no reliable microscopic theory on the vortex pinning due to disorder, in general (see W. Buckel and R. Kleiner, “Superconductivity”, Wiley-VCH, Weinheim (2004), p. 284).
From a physical point of view, the above pinning centers essentially produce the electric potential fluctuations, leading to the electron density fluctuations. Since the electron-phonon interaction, the deriving force of superconductivity, favors the electron density correlations (see Mi-Ae Park and Yong-Jihn Kim, “Weak localization effect in superconductors from radiation damage”, Phys. Rev. B 61, 14733(2000)), these pinning sites may favor the flux penetration, destroying superconductivity locally but allowing superconductivity overall, at high magnetic fields in type II superconductors. In short, these known techniques employ electric potential fluctuations, i.e., electric properties, to produce the pinning centers in superconductors. However, superconductivity near these pinning centers is destroyed due to the global energy minimization in the presence of flux penetration, rather than due to the local electric potential fluctuations. In other words, these potential fluctuations are not strong enough to destroy the superconductivity locally and therefore may not be the most desirable pinning centers.
In Magnetic Resonance Imaging (MRI) and Nuclear Magnetic Resonance (NMR) apparatus, the superconducting magnet forms a crucial part, because it dictates the operating temperature and the available magnetic field strength. Currently, Nb—Ti and Nb3Sn magnets are mostly used. However, the low transition temperature Tc of these materials requires liquid helium, which leads to the high cost of the MRI and NMR apparatus and maintenance. Therefore, it is highly desirable to find a higher Tc material for magnets of MRI and related NMR apparatus, which does not require expensive liquid helium.
In 2001, Akimitsu et al. found that MgB2 is a superconductor with Tc=39K (see J. Nagamatsu et al., Nature, volume 410 (2001), p. 63-64). Due to high Tc, low cost, and good mechanical properties, MgB2 has great potential for magnet applications. Indeed, in November 2006, the first MgB2 magnet-based MRI was introduced by ASG Superconductors, Paramed Medical Systems, and Columbus Superconductors. The operating temperature of 20K was achieved using two cryocoolers, without using any cryogenic liquid. However, the MgB2 magnet produced only 0.5 T, limiting the marketability of the MgB2 magnet-based MRI, because the current Nb—Ti and Nb3Sn magnet-based MRI can produce 3 T at 4K using the liquid helium. High magnetic fields provide better images because the resolution of the image depends on the square of the strength of the magnetic field (see E. M. Haake, R. W. Brown, M. R. Thomson, and R. Venkatesan, “Magnetic Resonance Imaging: Physical Principles and Sequence Design”, (Wiley-Liss, New York, 1999), p. 6).
For magnet applications, superconducting materials should have high critical current densities at high magnetic fields, due to strong vortex or flux pinning. However, MgB2 does not show good pinning properties. Therefore, additives can be introduced into MgB2 to enhance the vortex or flux pinning. For example, typical additives include: C (see S. X. Dou et al., Appl. Phys. Lett. 83, (2003) p. 4996), Al (see A. Berenov et al., Cond-mat/0405567 (2004)), SiC (S. X. Dou et al., Appl. Phys. Lett. 81, (2002) p. 3419), Ti, Zr (see Y. Zhao et al., Appl. Phys. Lett. 79 (2001) p. 1154), Si (see X. L. Wang et al., Physica C 385 (2003) p. 461), Y2O3 (see J. Wang et al., Appl. Phys. Lett. 81 (2002) p. 2026), and Mg(B,O) precipitates (see Eom et al., Nature 411 (2001) p. 558). Although considerable progress has been made, more progress is required for MgB2 to be used for high field magnets.